Investigation Of Tempering In A Medium Carbon Steel By Thermoelectrical Power

In the past several decades,in view of its importance in improving the strength and toughness of steels,tempering has received great concerns.Low temperature tempering has received attentions for the reason of transition carbides precipitated during early stages of tempering playing important role in improving strength and toughness of steels simultaneously.Though good combination of strength and toughness of steels can be obtained by low temperature tempering,the general road map of the tempering especially low temperature tempering is not clear.In this study,the thermoelectric power(TEP)was employed to research the low temperature tempering of a medium carbon alloyed steel.Meanwhile,X-ray diffraction analysis,micro-hardness,transmission electron microscopy and three dimensional atom probe were also used to investigate the low temperature tempering of a medium carbon steel,regarding the carbon distribution,the kinetics of segregation and clustering.Besides,we predicted and optimized tempering treatment through the TEP and combination of low temperature impact toughness,scanning electron microscope and so on.The main work of our study as follows:1.Investigation of carbon segregation during low temperature temperingIn this study,0.462C-1.319Si-1.706Mn-1.097Ni-0.354Mo(wt.%)steels were chose and tempered at 110℃,140℃and 170℃ for different time.From the variation of the TEP curves,the stages of carbon distribution during low temperature tempering such as dislocations from martensite,segregation and carbides precipitation are clearly evaluated.Additionally,carbon solubility variation can be calculated quantitatively during the tempering process,in this study,from 0.33 wt.% in quenching sample to a slightly stable 0.15 wt.%.Apart from the principle of time-temperature equivalence,carbon segregation to grain boundaries/ interfaces instead of dislocations are prone to occur at relative higher temperature,which may alter the mechanical properties when further tempering process is carried out for precipitation.Combination the TEP and hardness curves,support can be provided for choose an appropriate tempering temperatures and times.2.The tempering heat treatment optimization of a high strength steelIn this study,the tempering process of a 0.35C(wt.%)high strength steel was investigated by the thermoelectric power(TEP).According to the TEP results,optimized tempering temperature as estimated at 210℃.After heat treatments,low temperature impact toughness,hardness and scanning electron microscope experimental results confirmed the result indicated by TEP.Samples tempered at 210℃ for two hours have the highest impact toughness value of 9.33 J.Overall,the TEP can investigate the low temperature tempering and optimize the tempering process of medium carbon alloyed steels successfully.